A more modern disposal system is a septic tank with leech field (leach field). In a common septic-tank configuration, a home's wastewater fixtures are piped to a buried tank made of concrete or fiberglass. A discharge pipe leads away from the tank to a buried perforated pipe or network of pipes, known as a drainage field or leach field. Pipe elevations are maintained so that the septic tank remains almost full at all times, which permits bacteria to "digest" virtually all the solids that enter the tank. The remaining liquid is dispersed into the soil through the perforated pipes. These pipes may be laid in an area surrounded by gravel.
In locations where the soil cannot percolate the water away from the leech field properly, an ET (evapo-transpiration) or mound system may be employed.
The most common failure mode for septic systems occurs when the soil around the leech field becomes impervious to the flow of water. When this happens, the water level in the septic tank increases and plumbing fixtures in the house refuse to drain properly (sewage backs up). The ground around the leech field may also become odorous (septic tank odor), and ponds of standing water may appear, particularly in wet weather.
A variety of chemicals containing microbes, acids and enzymes are sold that attempt to open up the soil pores in the leech field and get the system working again.
Chemicals and mechanical methods can be used as a Do It Yourself method to keep a septic system working. If they prove inadequate the owner may need to call in a professional to evaluate the situation. Often, replacement of the entire leech field is required.